How Important Is Relieving Letter For Employement

I would like to know how important is the relieving letter for employing a new joinee.
Is it a necessity for the prospetive employee to provide the releiving letter.
What happens if he is not in position to provide Relieving letter or Letter of Accpetance. Is there any alternative approach?

Hi,
Reliev1ing letter is important at the the time of accepting new job because by this we can know that candidate in not committed to any other company and he is a genuine employee of that company.
Regards,
Malini